Generally, optical films, in particular retardation films, are used in display devices such as liquid crystal display devices and have functions such as color compensation, viewing angle expansion, and reflection prevention.
As the retardation film, λ/4 plates and λ/2 plates are known, and thermoplastic polymers such as polycarbonates prepared by polycondensation of bisphenol A, polyether sulfones, and polysulfones are used as materials for these plates. The λ/4 plates and λ/2 plates obtained by stretching films of these materials have a property of larger retardation at shorter wavelengths. Therefore, unfortunately, the wavelength at which the λ/4 plates and λ/2 plates can function is limited to a specific wavelength.
As a method of controlling a wavelength in a broad bandwidth, there is known a method of stacking two or more birefringent films, having different wavelength dependency of retardation, at specific angles (See, for example, PLT 1). In this case, due to use of a plurality of retardation films, steps of attaching the films and adjusting attaching angles are required, and thus, the productivity involves a problem. Moreover, as the thickness of the whole retardation film becomes large, its light transmittance lowers, resulting in an increase in the thickness and in the darkness when the film is integrated in an apparatus.
In recent years, there has been proposed a method of broadening the bandwidth using one film without such stacking (See PLT 2). This method includes a step of stretching a polymer film, wherein the polymer film is composed of a unit having positive refractivity anisotropy and a unit having negative refractivity anisotropy. However, the film specifically disclosed has a large birefringence due to a stress because of its high photoelastic constant, and has a problem of occurrence of light slipping when used as a retardation film. Furthermore, since an aromatic polycarbonate composed of a fluorene-based bisphenol skeleton is used, the film has a high melting temperature, thereby readily producing a gelled product through its decomposition when melt processed. Moreover, since the film has a high glass transition temperature (Tg), a high temperature is required for stretching the film and for steps like that, and special processing equipment different from that of the prior art is required and the like. From above reasons and the like, it cannot be said that its processability is satisfactory.
As films having a low photoelastic constant that can be produced by melt film forming, retardation films prepared by using a polycarbonate copolymer of 9,9-bis(4-hydroxy-3-methylphenyl)fluorene and an aliphatic diol and a polycarbonate copolymer of 9,9-bis[4-(2-hydroxyethoxy)phenyl]fluorene and isosorbide have been reported (see PLTs 3 and 4). However, there is no description about their durable stability, which was insufficient. Although a retardation film having a specific structure has been reported, its durable stability was still insufficient (see PLT 5). Moreover, the aforementioned PLTs have no description about significant enhancement in the durable stability by use of two kinds of fluorene-based monomers.